In the karst landscape of Quintana Roo, Mexico, plant access to water and nutrients is limited by heterogeneous soil pockets and conduits within the limestone bedrock. Because of the fine scale variability in bedrock characteristics, there can be substantial variability in water use and nutrient uptake that cannot be easily separated by plant traits or species groups. Despite this, the rather biodiverse forests in this region are classified as a medium statured semi-evergreen to evergreen, suggesting that there are several strategies to maintain plant performance among co-occurring tropical trees. We therefore assessed water use and resource acquisition of trees at four sites along the Caribbean coast of Quintana Roo during the dry season to uncover relationships between species, size, and site characteristics such as depth-to-groundwater and bedrock thickness. Within randomly established plots, species, diameter at breast height (DBH), and height were measured and noted for all trees with a DBH greater than five centimeters. Stem and leaf samples were collected from each individual and later analyzed for oxygen and hydrogen as well as carbon and nitrogen stable isotopes, respectively. Because each site was associated with a cave, we were able to assess bedrock characteristics, measure depth-to-groundwater, and collect groundwater samples.
Results/Conclusions
Results show significant variability in leaf carbon and nitrogen as well as stem water stable isotopes among sites and species. Leaf δ13C ranged from -35.3‰ to -27.4‰. With several legumes species occupying the lower end of the range, leaf δ15N ranged from -1.9‰ to 4.78‰. While the average δ18O of water in the caves and soil water was -1.2‰ and -4.5‰, respectively, stem water among individuals varied from -4.9‰ to -1.0‰. There is a striking relationship between leaf carbon and tree size. As trees become larger δ13C increases until a distinct size threshold in which δ13C then decreases. This suggests that the largest trees are not as water stressed compared to the medium sized trees. This is supported by a positive correlation between estimated deep water use and tree size. We noted that sites with shallower depth-to-groundwater or larger bedrock conduits allow for easier deep water access, potentially creating a division between shallow rooted and deep rooted species in terms of water and nutrient relations. This demonstrates that co-occurring tropical trees in Quintana Roo employ multiple resource acquisition strategies. Local site characteristics within the karst landscape seem to play a role in understanding this variability.